Conductivity, or electrolytic conductivity, is defined as the ability of a substance to conduct electrical current. Conductivity is the reciprocal of resistivity, and its basic unit is siemens per meter (S/m). For a liquid, the conductivity is a summation of contributions from all ions present in the liquid.
The measurement of the conductivity of liquids is usually performed in a so-called conductivity cell. Conductivity cells can be classified according to the technique by which they interact with the sample and also by the geometry of the cell design. The two operating techniques used are the contacting technique, and the inductive or toroidal technique. In both cases, an AC electrical input signal is applied to the conductivity cell, and the resultant AC electrical output signal from the cell is measured (a DC electrical signal would cause changes in the electrolyte).
Cells that measure conductivity using the contacting technique have their electrodes in direct contact with the liquid to be tested, whereas cells using the toroidal technique include two toroidal coils that are not in contact with the liquid.
The alternating current frequency typically ranges between 50 Hz and 50 kHz, depending on the electrolyte concentration of the liquid, the measurement frequency being increased with increasing ion concentration in the liquid in order to avoid interfering capacitance effects due to polarization effects at the electrodes.
A basic conductivity cell of the contact type has two parallel electrode plates, e.g. made from platinum. When an AC voltage is applied across the plates, an AC current will flow through the liquid which is inversely proportional to the electrical resistance between the electrodes, and thereby proportional to the conductivity of the liquid in the cell.
The resistance, R, of the cell is proportional to the distance, d, between the electrodes and the cross-sectional area, A, of the electrodes. The ratio d/A is usually referred to as the “cell constant” Θ, i.e. Θ=d/A.
For measurement on liquid flows, the conductivity cell is usually of flow-through type and coupled in-line with a liquid flow conduit. In this type of conductivity cell, the electrodes typically include two or more annular electrodes spaced along the stream of flow of the liquid. In this case, the cell constant will be defined by the ratio of the distance between the electrodes and the cross-sectional area of the conduit section between the electrodes.
The cell constant indicates the approximate range of conductivities that the cell can measure. In general, cells with a low cell constant should be used for measuring low conductivities, whereas a high cell constant is required for higher conductivity measurements.
U.S. Pat. No. 3,424,975 discloses a conductivity cell for measuring the electrical conductivity of flowing liquids in which the length of the electrical path, and therefore the resistance, of a conductivity cell can be varied to suit any particular use of the cell. In one embodiment, two parallel flow paths for the liquid are provided. One flow path is through three conduits in series which are formed of electrically conductive material. Conduits of electrically insulating material extend from the central conductive conduit into the outer conductive conduits to provide elongated electrical paths between the central conductive conduit and the outer conductive conduits. A valve is provided in the second flow path to permit relative adjustment of flows through the two paths.
U.S. Pat. No. 5,441,049 discloses a conductivity cell having a passageway through which liquid flows with a constriction in the passageway for regulating the flow and providing a predetermined cross-sectional area for measuring conductivity. Electrodes, which are preferably cylindrical, having their longitudinal axes parallel with the passageway, are located on each side of the constriction. For such a conductivity cell having a given length and outside diameter, a cell constant in the range from one to one hundred can be obtained by varying cell parameters including inside diameter at the constriction, inside diameter at the ends, inside diameter at the electrodes, and center-to-center spacing of the electrodes.
In many process flow systems, e.g. cross-flow filtration systems, a low hold-up volume is highly desired. However, when scaling up a conductivity cell type designed for laboratory applications which has a certain cell factor, to the piping diameters used in process flows, keeping the same cell factor will considerably increase the length of the conductivity cell and result in undesired large hold-up volumes. Conductivity cells of the prior art types mentioned above do not provide any solution to this problem.
It is therefore an object of the present invention to provide a conductivity cell for large diameter piping applications, such as bio-process flows, which overcomes the above-mentioned problem of increased cell length and hold-up volumes.